The in vitro cultivation of the filarial nematode Brugia malayi from the infective stage to the fourth and the young adult stage is described. Different culture conditions including cell-free systems and coculture with different human lymphatic cell lines were compared. Cocultivation with each of the cell lines used enhanced the survival of the parasites. The best results were obtained employing the human T cell leukemia line Jurkat and human dermal fibroblasts as feeder cells in RPMI 1640 supplemented with 10% heat-in-activated human serum. This culture system allowed the cultivation of B. malayi for more than 7 weeks with an average growth of the larvae by factor 6.4 (0.77 +/- 0.035 cm) and a maximum growth by factor 10 (1.2 cm). 100% of the parasites reached the fourth larval stage after 14 days. By day 37 of culture, 17% of the larvae had completed their final moult. Parasites remained alive up to 52 days. During the first four weeks of culture, both the length and the periods of moulting of the in vitro cultivated filariae closely resembled those observed with B. malayi in vivo in rodent hosts. This system provides a new approach for successful cultivation of B. malayi, that should help to overcome the difficulties of reproduction by other laboratories encountered with previous culture systems. It appeared highly reproducible in our hands and possibly further improvement can be obtained on its basis. This cultivation system represents a further step towards the complete replacement of the vertebrate host by means of in vitro culture systems in parasitological research.